In order for the SEI to work, you have first to get ride of the space shuttle, and freedom space station. none of the two can be really used for moon or mars.
Problem = the shuttle flew in 1981 and cost billions. As of 1989, you can't threw it in the dust bin (today we can !)
You can't get ride of Freedom (1984) either... even if it does not exist in 1989.
How weird.
In fact ESA and Japan were already involved (since 1985-1987) through agreements such as the Columbus module. Killing these agreements would be devastating. It nearly happened in 1993, and NASA prefered instead "reborn" Freedom as the ISS we have today.
Beyond the Shuttle / Freedom conundrum are more teething problems.
- NASA administrator is Richard Truly, an astronaut... and a shuttle pilot. He was a disaster as administrator.
- The SEI was imagined at the Johnson Space Center, by Aaron Cohen. An honorable man, alas from the Apollo era (Ie Bush-is-like-Kennedy, cost-is-not a-problem philosophy)
In order for the SEI to work you have first to cancell the shuttle in the early 70's, replace it by a capsule (keep Apollo !) then build "Freedom" in 1980-1986, from Skylab experience and hardware.
Once space-buff Bush is elected in 1989, he found the station already in orbit, and a capsule that NASA can modify for lunar or Mars missions. That's better to start from.
Next problem is cost and administrator. Forget Truly, try someone like Dan Goldin and its "faster,better, cheaper" approach.
Dan Goldin and Bob Zubrin know each other quite well; Mars direct and "faster better cheaper" might work well together.
However a Mars landing is very expensive, difficult and risky. Bush planned it for 2019, not 1999.
However you can have a Mars mission for 1999... if you accept not landing on the red planet.
Here's Zubrin takes on a cheap, pathfinder Mars flyby mission.
(Athena, dated from 1996 !)
http://pdf.aiaa.org/getfile.cfm?urlX=85%26%5D0%3BU%2BDN%26S7R%20CMU%24CBQ%3A%2B64K8%26%5FOGJ%0A&urla=%25%2ARH%27%21P%2C%20%0A&urlb=%21%2A%20%20%20%0A&urlc=%21%2A0%20%20%0A&urle=%27%2B%22D%22%23PJCU0%20%20%0A
Athena is truly interesting. It is cheap, and small enough to ride onto a single Energia... or four Ariane V or Proton medium-lift boosters!